feat(loyalty): multi-select categories on transactions

Switch from single category_id to category_ids JSON array on
transactions. Sellers can now select multiple categories (e.g.,
Men + Accessories) when entering stamp/points transactions.

- Migration loyalty_009: drop category_id FK, add category_ids JSON
- Schemas: category_id → category_ids (list[int] | None)
- Services: stamp_service + points_service accept category_ids
- Terminal UI: pills are now multi-select (toggle on/off)
- Transaction response: category_names (list[str]) resolved from IDs
- Recent transactions table: new Category column showing comma-
  separated names

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/services/card_service.py

@@ -836,8 +836,8 @@ class CardService:
                 "transaction_at": tx.transaction_at.isoformat() if tx.transaction_at else None,
                 "notes": tx.notes,
                 "store_name": None,
-                "category_id": tx.category_id,
-                "category_name": None,
+                "category_ids": tx.category_ids,
+                "category_names": None,
             }
 
             if tx.store_id:
@@ -845,15 +845,19 @@ class CardService:
                 if store_obj:
                     tx_data["store_name"] = store_obj.name
 
-            if tx.category_id:
+            if tx.category_ids and isinstance(tx.category_ids, list):
                 from app.modules.loyalty.services.category_service import (
                     category_service,
                 )
 
-                cat_name = category_service.validate_category_for_store(
-                    db, tx.category_id, tx.store_id or 0
-                )
-                tx_data["category_name"] = cat_name
+                names = []
+                for cid in tx.category_ids:
+                    name = category_service.validate_category_for_store(
+                        db, cid, tx.store_id or 0
+                    )
+                    if name:
+                        names.append(name)
+                tx_data["category_names"] = names if names else None
 
             tx_responses.append(tx_data)